Systems for high frequency communications typified by mobile phones have used high-frequency filters to remove unnecessary signals other than the frequency band used for communication. Acoustic wave resonators such as surface acoustic wave (SAW) resonators have been used for the high-frequency filters. In the SAW resonator, an Interdigital Transducer (IDT) including a plurality of electrode fingers is formed on a piezoelectric substrate such as a lithium tantalate (LiTaO3) substrate or a lithium niobate (LiNbO3) substrate. The region where the electrode fingers of the IDT overlap is an overlap region. The IDT excites a Shear Horizontal (SH) wave (leaky wave), a Rayleigh wave, or a boundary acoustic wave that is a type of the surface acoustic wave. Reflectors located at the both sides in the main propagation direction of the acoustic wave excited by the IDT confine the acoustic waves around the IDT. The use of the acoustic wave resonators allows for making of a ladder-type filter and a multimode filter.
International Publication No. 2015/007319 and U.S. Pat. No. 7,939,987 disclose that regions with different acoustic velocities of acoustic waves are periodically arranged at regular intervals in the extension direction of the electrode fingers in the overlap region.
When the regions with different acoustic velocities of acoustic waves are arranged at regular intervals, lateral-mode spurious can be reduced. However, the degree of reduction of the lateral-mode spurious is insufficient.